3. Fire-Resistant Steel

This chapter is concerned with the design and characterisation of fire-resistant steels for building construction. Issues raised include effects of alloy additions, controlling the grain size, properties of substitutional elements, and processing, centred on the property requirements for fire-resistant steels. The new fire-resistant steels are microalloyed with molybdenum and niobium, or tungsten, titanium and boron, as the alloying additions for elevated temperature strength. The steels have satisfactory high temperature strength, owing partly to their relatively large grain sizes compared with conventional steels. The nature of equilibrium precipitation is calculated using thermodynamics. The high temperature strengthening mechanisms could be attributed to secondary formation of fine distribution of MC and Laves phase precipitates, molybdenum clusters, and molybdenum and niobium in solid solution. The atomic-scale microstructure of structural steels designed to have fire-resistant microstructures is characterised using atom probe field ion microscopy (APFIM). There is strong precipitation at approximately 650 °C. These probably coherent precipitates have the effect of producing a stable dispersion.